This invention relates to a novel in-line melt rheometer capable of producing reproducible and accurate measurements of polymer rheology, and especially of apparent polymer viscosity at practical shear rates.
Knowledge of polymer rheology is very important for efficient design and monitoring of polymer melt-processing operations. Polymer melt viscosity affects its rate of flow as well as its ability to fill molds. Since polymer rheology is dependent on several factors, including the molecular weight of the polymer, melt morphology, and physical structure, it is a property suitable for monitoring possible changes of molecular weight, changes in melt elasticity, and changes of morphology of the polymer during melt processing, so that processing conditions can be adjusted accordingly.
While the importance of accurate rheology measurements during melt processing has always been recognized, and some equipment was available for determining polymer melt viscosity, especially in sampling side streams, there has heretofore existed no practical method or fast response equipment for determining in-line melt viscosity of polymers, especially of filled, e.g., fiber-reinforced polymers and of other highly viscous polymer compositions over a broad range of shear rates, especially at high shear rates. Normally, a sample of such a composition had to be removed from the melt processing equipment such as, for example, an extruder and tested in a melt indexer or other suitable apparatus. This normally was a long operation, so that there always was a time lag between polymer sampling and melt viscosity determination. This naturally had the shortcoming of making it impossible to adjust melt processing conditions as soon as a departure from the desired predetermined standard occurred. Equipment used for side stream rheology measurements had slow response of about 10 minutes or more and, therefore, could not provide rapid feedback for accurate process control. Besides, such equipment was not measuring properties of a truly representative process stream.
There has always existed, therefore, a great need for a practical means for quickly and accurately determining the melt rheology of polymers during their melt processing.